In order to detect connection conditions of a secondary battery connected to a charging device, such as a lithium-ion battery, usually a thermistor installed in the secondary battery can be used to detect the temperature of the secondary battery being charged, or a built-in resistor can be used to detect the connection conditions of the secondary battery. Specifically, a current is directed to flow through the thermistor or the resistor, and a voltage occurring between two ends of them is measured to detect the battery connection conditions. Alternatively, as a well known method, a current is directed to flow through the secondary battery connected to the charging device, and a voltage between two battery connection terminals of the charging device can be measured to determine whether the battery is reliably connected.
In the former method, when the secondary battery is connected to the charging device, even if the terminals of the thermistor or the resistor and the charging device are reliably connected, it cannot be guaranteed that the electrode of the secondary battery and the charging device are reliably connected. In other words, even when the charging device charges the secondary battery assuming the secondary battery is reliably connected, sometimes, because the battery connection terminal actually is not in contact with the electrode of the battery, the secondary battery is actually not charged. Similarly, even when the electrode of the battery is connected to the charging device, if the thermistor or the resistor is not in good connection condition, temperature control cannot be carried out appropriately in the course of charging, insufficient charging happens, and the secondary battery may experience problems.
FIG. 7 is a block diagram showing an internal configuration of a charging device 100 of the related art, presenting a method of confirming the voltage on a secondary battery BAT mounted on the charging device 100.
As illustrated in FIG. 7, a charging control circuit 101, which functions as a detection circuit for detecting whether the secondary battery BAT is reliably connected, includes a preliminary charging constant-current circuit 102 for supplying a predetermined current to the secondary battery BAT, a diode Da for preventing a reverse current, a battery pack voltage detection circuit 103 for detecting the voltage on a battery connection terminal Ta of the charging device 100, a current detection circuit 104, and a charging FET control circuit 105. A resistance component r corresponds to the sum of the internal resistance of the secondary battery BAT and the contact resistance between an electrode of the secondary battery BAT and the battery connection terminal Ta of the charging device 100.
When the secondary battery BAT is reliably connected, the impedance of the resistance component r is small, and it is thought that the change of the voltage caused by the resistance component r is negligible, hence, the voltage on a battery connection terminal Ta equals the voltage on the secondary battery BAT. However, the impedance of the resistance component r increases if the secondary battery BAT is not appropriately connected, or even though the secondary battery BAT is appropriately connected, the electrode of the secondary battery BAT and the battery connection terminal Ta of the charging device 100 are not in good connection conditions, resulting in a large contact resistance between them, or the secondary battery BAT is not in good connection conditions, and has a large internal resistance.
In these cases, even when a small current is supplied to the secondary battery BAT, the voltage on the battery connection terminal Ta of the charging device 100 increases up to the voltage on an input terminal (IN) connected to an AC adapter. From these facts, the battery pack voltage detection circuit 103 detects that the voltage on the secondary battery BAT reaches an un-specified value, and with the detection results, it is possible to reliably connect the battery.
For example, Japanese Laid Open Patent Application No. 5-219656 discloses a battery charger which has a charging mode and an adapter mode, and when bad connection conditions or other anomalies are detected, the battery charger shows an alarm and stop output.
The charging device of the related art, however, is supplied with a DC current from an AC adapter, which works as a power supply. Due to this, high frequency noise superposed on the AC line is also superposed on the power supply voltage Vdd. Namely, the high frequency noise also influences the battery connection terminal Ta, and sometimes, the high frequency noise may also be superposed on the voltage on the battery connection terminal Ta. When the voltage superposed with the high frequency noise reaches an un-specified value for the secondary battery BAT, a determination is made that bad connection conditions occur. In addition, when the voltage of the AC line decreases depending on the load of the charging device 100, the voltage on the battery connection terminal Ta does not reach a battery pack unspecified value inside the charging device, and this cannot be determined as bad connection conditions.